It is well known that in three-phase single-core cable circuits, the EMF will be kept to a minimum when the cables are placed in a trefoil configuration, in contrast to a flat configuration, which is much less favourable. A reduction of the inter-phase distance will also reduce the EMF, but thermal conditions will usually limit this distance to some minimum value.
As acceptable EMF exposure levels have decreased dramatically in many countries, it is often necessary to apply a passive EMF shield encompassing the trefoil cable arrangement for high-current electric power circuits. Such EMF shields will normally be composed of vertically and horizontally oriented steel, copper or aluminium plates. An example of such a shielding method is found in WO 2004/034539.
An alternative to the plate shielding referred to above, could be to apply massive metallic screens on the single-core cables. For three-phase cables this has been described in WO 99/44208 where a braided sheath or screen surrounds the electric power conductors so as to provide a shielding effect. Significant screen currents may be induced in particular with the first alternative referred to, thus necessitating a large screen cross-section so as to limit the additional power loss that might be a result of the screen current.
Another known proposal employs a six-phase system and six single-core cables. This solution (Brakelmann: Elektrizitätswirtschaft, year 94 (1995), No. 15, EMV-Massnahmen für Drehstrom-Einleiterkabel) requires special transformers at both ends of the (shielded) cable circuit to produce a symmetric six-phase system.
Both the use of massive metallic screens and the use of metallic plate shields are costly and will introduce additional power losses. Furthermore, the function of plate shields is dependent upon a stable geometry and of course durability of the metallic plates. The shielding effect may be reduced over time in case of corrosion and/or soil shifting, that may lead to detrimental cracks or grooves in the plate material.
Reference is further made to U.S. Pat. No. 6,506,971 describing various conductor arrangements in multi-core cables, but these are not directed to three-phase single-core-cables in a system of the type contemplated here. Similar comments apply to Russian patent publication RU2025014.